<style>
	button{position:absolute;}
	button#b1 {
		top:0; left:50%;
		nav-index:1;nav-right:#b2; nav-left:#b4;nav-down:#b2; nav-up:#b4;
	}
	button#b2 {
		top:50%; left:100%;
		nav-index:2;nav-right:#b3; nav-left:#b1;nav-down:#b3; nav-up:#b1;
	}
	button#b3 {
		top:100%; left:50%;
		nav-index:3;nav-right:#b4; nav-left:#b2;nav-down:#b4; nav-up:#b2;
	}
	button#b4 {
		top:50%; left:0;
		nav-index:4;nav-right:#b1;nav-left:#b3;nav-down:#b1; nav-up:#b3;
	}
</style>
<div>
	<button id="b1">按钮一</button>
	<button id="b2">按钮二</button>
	<button id="b3">按钮三</button>
	<button id="b4">按钮四</button>
</div>
<script>
	function a(){
		alert(0)
	}
</script>